Engine shake

[Beaver77]

On my swift 44 the port engine has a little visible shake between 800-1400rpm then smooths out above that….that engine also usually has one to two lbs more boost than the starboard for a given rpm which I have attributed to the props not balanced which I intended to fix….the engine shake I can also see a slight wobble of the shaft where it goes into the seal at that rpm, I guess it is slightly out of alignment or maybe related to the prop? Only had the boat 6 months but on the survey the props looked fine thanks

 

[Comodave]

The easiest thing to check is engine alignment. Pretty easy thing to do.

 

[DennisB1]

Your engine could be out of alignment or you have a bent shaft. Check both. Requires feeler gages and dial indicator to make measurements.
Also could be engine problems. Injectors or valve adjustments. Check the alignment first which is best down with boat in the water.

 

[C lectric]

Prop dings can cause shaking. It is often hard to see those dings unless the prop is on a bench or use a stand to hold a measuring tape next to the prop when out of the water. Usually requires a second helping hand pr two,
Shaft can be watched for any sign of misalignment which shows as wobble or shaking.

 

[Lepke]

A bad alignment can ruin the aft bearing in the transmission and possibly other parts.

 

[Steve DAntonio]

In my experience shaft/engine misalignment, because it is constant, does not cause vibration or shaking. Vibration and shaking can be caused by several things including...

• Prop damage or fouling
• Shaft damage/bend
• Props not installed on shaft properly
• Shaft-coupling or shaft coupling to output coupling non-concentricity (it would have been this way from the start). Described here https://stevedmarineconsulting.com/wp-content/uploads/2019/06/ShaftCouplings179FINAL-PBB.pdf
• Defective, worn out, collapsed motor mount(s)
• Misfiring cylinder (rare for this to be only at a narrow rpm)

 

[RickyD]

My buddies recently bought a boat that had a bad shaft vibration. Turns out one of his struts was almost an inch off center. Don that could be. He had the strut moved and now its perfect.

 

[TBill36]

Motor mounts can be loaded incorrectly and still show good shaft alignment. Cattycorner mounts can position the engine correctly with little loading on the opposing mounts. Then when the engine is loaded it rocks on the cattycorner mounts until the opposing mounts stop the movement, hence the vibration. Some mounts provide a feature to determine how much it is loaded. Mine have a hole on the underside of the mount where you insert a pin to determine the clearance between underside feature of the mount and the stringer it sits on. The internal rubber construction of the mount compresses as it gets loaded and decreases the clearance as it gets loaded. The goal is to get all mounts to have the same clearance or at least the front 2 and back 2 should be as close as possible.

 

[O C Diver]

On my swift 44 the port engine has a little visible shake between 800-1400rpm then smooths out above that….that engine also usually has one to two lbs more boost than the starboard for a given rpm which I have attributed to the props not balanced which I intended to fix….the engine shake I can also see a slight wobble of the shaft where it goes into the seal at that rpm, I guess it is slightly out of alignment or maybe related to the prop? Only had the boat 6 months but on the survey the props looked fine thanks

Did it have the shake during seatrial? Does it shake at the same RPM out of gear? Are you in salt or brackish water (growth on the propeller)? Lots of possibilities.

Ted

 

[DrissZougari]

Sometime a dirty fuel filter would cause the engine to vibrate similar to a bent prop!

 

[lwarden]

I'll add one more potential cause:
I had a weak injector that caused a shake under load. A new set of injectors solved that issue.